What do I do with the half I didn’t use?

Half an avocado, brown by morning. A block of cheese with a green bloom on the cut face. Half a can of tomato paste growing a skin for the fifth time this year. Seven pages in this library are about the half that gets thrown away. They agree on a small number of moves, and they carry more safety lines than you might expect from a question about leftovers, so those are kept whole here.

Keep the air off it and put acid on it. Press the film onto the guacamole is built from thirty conversations and 396 accounts about a cut avocado. Nine separate conversations say it is air that browns the flesh, and nine say citrus slows it, so everything else on that page is a way of getting between the flesh and the air: cling film pressed flat onto the surface so no air sits between film and food, the cut face laid on a plate, the two halves pushed back together, the lime first. The pit only shields what it touches, one correction backed by two accounts says. Five separate conversations keep a cut half under water overnight and the page does not, because one account and the US Food and Drug Administration, in a 2022 statement reported by a US news outlet, say the water is where bacteria grow. The FDA was answering a fashion for keeping whole avocados under water for weeks: any Listeria or Salmonella on the skin can multiply in the water, and its research found Listeria can move into the flesh within fifteen days of refrigerated submersion. The FDA’s produce page also says to wash all produce under running water before cutting, even when you will not eat the skin, so the knife does not carry dirt from the skin into the flesh. One caution on that page keeps everyone honest: no method is perfect, air gets in, and the difference is between a thin brown skin and a brown half.

Cheese goes mouldy from your hands before it goes mouldy from the wrap. Keep your fingers off the cheese draws on twenty-three conversations and 388 accounts. Four separate conversations say touching the cheese leaves oils and bacteria on it that speed the spoilage, and one account says that matters more than which wrap you use; one correction, two accounts behind it, says the shop wrapper left on inside a bag traps moisture and bacteria against the cheese, so throw it away and wrap the block fresh. Paper or plastic is the argument that page does not settle: six separate conversations say paper so the cheese can breathe, three answer that cheese counters heat-seal plastic and a bag lasts weeks, and the accounts’ own decider is the cheese, hard or soft, how long it is being kept, and whether the plastic is sealed tight or left loosely rolled. Mould already on the block is a gap in those conversations, so the page carries the US Department of Agriculture’s guidance as widely reproduced today, because its own page could not be reached, and the words of the director of the University of Georgia’s Center for Food Safety, and the rule is worth reading whole. For a hard cheese, cut off at least an inch, 2.5 cm, around and below the mould spot, keep the knife out of the mould so it does not drag spores across the fresh cut, and rewrap in new paper or a new bag; throw the whole thing away if the mould covers the surface or the piece is too small to cut an inch below it. Soft cheeses, which that page lists as cottage, cream cheese, ricotta, brie and camembert, and any shredded, crumbled or sliced cheese with mould on it, are thrown away, not trimmed, because the moisture lets the mould grow deeper and spread its toxins further. The fridge line is the authorities’ too: at or below 40°F, 4°C, in the FDA’s words, and between 0 and 5°C in the UK government’s. The grater trick on that page is thirty minutes of chill, not a night in the freezer; one correction, three accounts behind it, says frozen-solid cheese crumbles under the grater.

Freeze the rest in spoon-sized portions. Zest before you juice, then freeze it in cubes comes from thirty conversations and 421 accounts with soft lemons, blackening herbs and half a can of paste, and for each of the three, four separate conversations freeze it in portions. Lemon juice into an ice-cube tray, then into a bag, because two cautions say juice left in an open tray takes freezer burn and off-flavours; one correction backed by four accounts says zest first, because a squeezed lemon is too soft to zest, and a prevalence note says juicing first is the common mistake. Herbs chopped and topped with olive oil or melted butter in the same tray, one cube per pan. Tomato paste in tablespoon heaps on a sheet of baking or wax paper, frozen flat and broken into a bag; one correction on that page explains why the can was always too big, because paste is concentrated and a recipe wants a tablespoon or two. Six separate conversations, the widest agreement on that page, cut fruit that is about to turn, spread it on a tray so the pieces freeze apart, and use it in smoothies in place of ice. One line stands above the method there, checked for the page: two cautions rated high and one contraindication say that garlic, and to a lesser degree herbs, sitting in oil at room temperature can grow the organism that causes botulism, and US food-safety guidance says homemade garlic-in-oil must be kept in the fridge and used within a few days, or frozen, never left out at room temperature. And one contraindication, three accounts strong: do not store tomato paste or other acidic food against aluminium foil, because the acid eats through it and the food picks up the metal.

Stale is not the same as dry, so put the water back. Wet the bread before it goes back in is built from thirty conversations and 406 accounts. Four separate conversations revive hard bread with a spray, a mug of water standing beside it or a damp paper towel and a short burst of heat, because staling is the starch giving up its moisture; four say the oven does it better than the microwave, and that argument is carried three accounts to three, with the decider being what you are reviving and whether you want the snack this minute or the texture back properly. Microwaved bread has to be eaten at once; one caution says it turns near-inedible once it cools again. Soft crisps need no water: tip them out onto a plate, twenty to thirty seconds, and let them cool before you judge them. Past reviving, six separate conversations dry it for crumbs and croutons and five soak it for French toast or bread pudding, because dry bread drinks the custard. Four cautions on that page are about fire, the accounts’ own and rated high, and they belong here whole. Never microwave the crisp bag itself, because it can spark or shrink. Do not microwave heavily seasoned chips; one account says a cumin-dusted batch caught fire. Keep recycled paper towels out of the microwave, because they can carry tiny metal flecks that spark. Take the twist tie off the bread bag before anything goes near the microwave. And mouldy bread is a different thing from stale bread: the one account that addresses it says soft foods like bread with mould on them go in the bin whole, and the page carries no trimming rule for bread.

Cook the whole bag, then freeze what you cooked. Cook the whole bag, then freeze it draws on thirty conversations and 445 accounts with a big cheap bag of potatoes in a warm kitchen. Four separate conversations say the way through it is to cook a large batch, mashed or baked are the two they name, and freeze it, so the week becomes reheating instead of peeling. The uncooked ones want somewhere cool, dark and dry, around ten degrees Celsius in one account’s detail, and away from bananas, onions, apples and avocados, which hurry them along. On the fridge, the page names both sides because the world moved: US food-safety guidance still says do not refrigerate raw potatoes, because the cold turns some of the starch to sugar and fried potatoes then form more acrylamide, and UK food-safety guidance changed in 2023 after its own research and now says the fridge is fine. Two safety lines sit above that page. Dry beans, which four separate conversations put with a potato to make it a meal, carry a natural toxin that boiling destroys; public food-safety guidance in the US and the UK names the kidney bean, says raw or undercooked ones cause severe vomiting and diarrhoea within hours, and says to soak them, throw the water away, and boil them hard, at least thirty minutes in the US guidance and at least ten in the UK’s, before they go into a slow cooker or a stew, because a slow cooker on its own may not get hot enough and undercooking can make the beans worse. And three separate conversations, a contraindication and a caution rated high say a diet of only potatoes runs out of vitamin B12; the page is for stretching a bag, not for living on one food. Food-safety guidance adds that the green and the sprouts on a potato contain solanine, so cut them away and throw out any potato that is very green or tastes bitter.

Set honey has not gone off, and the jar goes in warm water, not boiling. Warm the jar, don’t boil it exists because one account says people throw away honey that has gone cloudy and hard, and two corrections put it plainly: pure honey crystallises, and one correction adds that it is, if anything, a sign of the real thing. Five single accounts stand the glass jar in warm water and wait; a correction, and an argument settled two accounts to one, says boiling is counterproductive because it makes the honey set again faster. Two lines on that page are not preferences. Never heat honey in its plastic squeeze bottle, not in the microwave, not in hot water, not in a dishwasher with a heat cycle, because two warnings and four cautions say the plastic can leach or melt; decant into glass first, and glass has its own rule, no sudden temperature change, or the jar cracks. And never give honey to a baby under one year old, including honey stirred into food, because honey can carry spores that cause infant botulism. Or, as three and four accounts on that page say, do not bother melting it: set honey spreads, and some people prefer the texture.

The brine is an ingredient, once. Refill the pickle jar says the liquid left when the pickles run out is a finished, seasoned brine with at least one more job in it: washed, thin-sliced cucumber dropped in, the jar back in the fridge, and in two or three days there are new pickles, milder than the ones you bought. The envelope on that page is short and not optional. The refilled jar lives in the fridge and gets eaten within a week or two; the liquid gets reused once, not over and over, because every batch of vegetables waters it down and the acid and salt are what keep it safe; it never goes on a shelf and is never a canning liquid; and started clear but turned cloudy, fizzing, or smelling wrong means the bin, not a taste. The page also brines chicken in it, with one settled rule from those exchanges that twenty-four hours is the ceiling.

Who this is not for, and what these pages leave open. The food pages cannot tell you how many days each method buys; “next day”, “a couple of days”, “weeks” and “months” are the accounts’ own words, and the avocado, cheese, lemon and potato pages each name keeping time as a gap; the pickle page is the one that gives times, two or three days to new pickles and a week or two to eat them. If your cheese is brie, cottage or a bag of grated, the trimming line is not for you and the block goes. If you want lemon slices for a glass, the freezer is not your answer, because the flesh goes soft. If you are feeding a child under one, the honey page says the rule again on purpose. And if the half you cannot use is a symptom of not having enough for the whole, the second follow-up below is this library’s page for that.